AGP was a modified PCI to help videocards. It offered extra options like the possibility to increase the speed of the BUS (agp 1x, 2x, ...) and it offered some extra options. The pci express bus is a totally new more universal bus. It doesn't need extra options like this I think. It might be possible to tweak some pci express things in the "config space" but there's nothing for it yet. (you can do similar things for normal pci cards but useally it doesn't do that much; think of things like adjusting latencies)